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1. Field of the Invention
The present invention relates to a process for preparing propene and, if desired, 1-butene by metathesis of olefins.
Olefin metathesis (disproportionation) in its simplest form describes the reversible, metal-catalyzed rearrangement of olefins by cleavage and reformation of Cxe2x95x90C double bonds. For example, olefins of the formulae R1xe2x80x94CHxe2x95x90CHxe2x80x94R2 and R3xe2x80x94CHxe2x95x90CHxe2x80x94R4 are reversibly reacted to form olefins of the formulae R1xe2x80x94CHxe2x95x90CHxe2x80x94R3 and R2xe2x80x94CHxe2x95x90CHxe2x80x94R4. In the metathesis of acyclic olefins, a distinction is made between self-metathesis in which an olefin is converted into a mixture of two olefins having different molar masses and cross0- or co-metathesis in which two different olefins react. An example of self-metathesis is the reaction of two molecules of propene to give ethene and 2-butene, as is performed, for. example, by the Phillips triolefin process, see Hydrocarbon Processing, Volume 46, November 1967, No. 11, p. 232. An example of cross-metathesis is the reaction of propene and 1-butene to give ethene and 2-pentene. If one of the reactants is ethene, the reaction is customarily referred to as an ethenolysis.
The metathesis reactions are carried out in the presence of catalysts. Suitable catalysts for this purpose are, in principle, homogeneous and heterogeneous transition metal compounds, in particular those of transition groups VI to VIII of the Periodic Table of the Elements, as well as homogeneous and heterogeneous catalyst systems in which these compounds are present.
DE-A-19 40 433 discloses the metathesis of 1-butene with 2-butene to form propene and 2-pentene, with Re2O7/Al2O3 being used as catalyst. The 2-pentene formed is reacted further with sodium hydride on potassium carbonate and ethene to give heptenes.
The metathesis of 1-butene and 2-butene to give propene and 2-pentene is mentioned in K. L. Anderson, T. D. Brown, Hydrocarbon Processing, Volume 55, August 1978, No. 8, pp. 119-122 as a secondary reaction in the synthesis of isoamylene.
EP-A-0 304 515 discloses a metathesis process for reacting 1-butene with 2-butene to give propene and pentenes, which is carried out in a reactive distillion apparatus using Re2O7/Al2O3 as catalyst.
U.S. Pat. No. 3,526,676 discloses the metathesis of 1-butene with 2-butene to give propene and pentene. It is carried out over MoO3 and CoO on Al2O3.
U.S. Pat. No. 3,785,957 discloses a process for the production of fuel having a high octane number. In this process, an olefinic fuel is disproportionated together with ethylene, the product is fractionated into a propylene stream, a butene stream, a C5- or C5-C6-olefin stream and a C6+ or C7+ fuel stream. The C5- or C5-C6-olefin stream is disproportionated with ethene over a WO3/SiO2 fixed-bed catalyst to give propylene and butenes. The propylene obtained is disproportionated to form ethylene and butenes and the butenes are alkylated with isobutane.
U.S. Pat. No. 3,767,565 discloses a process for increasing the octane number of fuel in which a C5 fraction of an olefinic fuel is reacted with ethylene in the presence of a catalyst comprising WO3/SiO2 and MgO to form ethylene, propylene, n-butenes and isobutenes. The propylene obtained is disproportionated and the resulting n-butenes are alkylated with isobutane.
EP-A-0 691 318 discloses an olefin metathesis process in which C5-olefins and ethylene are reacted in the presence of a catalyst to give mixed C4-olefins and propene. Thus, 2-methyl-2-butene is reacted with ethene to give isobutene and propene. A mixture of 2-pentenes and 2-methyl-2-butene is reacted to give a mixture of 1-butene, isobutene and propene.
2. Description of the Related Art
In the above processes, propene is prepared with addition of at least equimolar amounts of ethene. To achieve high propene selectivities, a large amount of ethene has to be circulated.
It is an object of the present invention to provide a process for preparing propene and, if desired, 1-butene as coupled product in variable amounts using C4-olefins such as raffinate II.
We have found that this object is achieved by a process for preparing propene and, if desired, 1-butene by
a) reacting 1-butene and 2-butene to give propene and 2-pentene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group VIb, VIIb or VIII of the Periodic Table of the Elements,
b) subsequently separating the propene and 2-pentene formed,
c) subsequently reacting the 2-pentene with ethene to give propene and 1-butene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group VIb, VIIb or VIII of the Periodic Table of the Elements,
d) subsequently separating the propene and 1-butene formed,
e) discharging at least some of the 1-butene formed and/or at least partly isomerizing the 1-butene formed to give 2-butene in the presence of an isomerization catalyst and subsequently returning the undischarged 1-butene and the 2-butene formed together with a part of the C4 fraction not reacted in step a) to step a).